The problem is that in my case there is no dedicated IP.
The VPN is using a dynamic IP which changes everyday.
Therefore it uses a no-ip.org name.
From what you are saying, the dynamic address is for the VPN Connection. In the RDC connection, you would not be entering that, you would be entering the LAN address of the SERVER, not the VPN.
I have had no issues connecting from MAC RDC to any servers I have tried, but I did have some issues when I first switched to Mac. I found that when I was on the LAN, I could always connect, but when trying to connect remotely through VPN, I would have trouble. So, I learned to use the FQDN (myServer.MyDomain.net) of the remote system instead of using just the server name (myServer).
The VPN client you are using may be a factor in this, but I found that the built in client I started with didn't automatically route all lookups through the vpn tunnel - but if I used the FQDN, it did route properly and find the internal address of the server.
Also, if you can connect to win 2K3 servers but not Win2K8 on the same remote network, there may be a policy setting on the server that requires the client to provide some identification that isn't supported by the MAc version of RDC. I know that Win2K8 added some security options, but I don't remember exactly what they were. The Mac RDC client definitely does not support everything that the Windows client does, but you should be able to at least get connected.
The two features that I notice missing on the Mac Client are : Use all Monitors and smartcard passthrough. For Smartcard passthrough, I have to run a Fusion VM and launch the Windows RDC client from there...